At present, there are no millimeter-wavelength (terahertz-frequency (THz)) sources that are compact, light-weight, narrow-linewidth, tunable, and high-power sources. Referred to hereafter as THz sources, these sources output electromagnetic radiation at a frequency in the range of at least about 0.1-10 THz.
Conventional sources in the terahertz (THz) range (i.e., 0.1-10 THz) have either too-low power or are only available in limited wavelengths and frequency bands. FIG. 1A shows the power- and frequency-capability characteristics of various conventional THz sources. 1—PCA (photoconductive antenna; e.g., interdigitated PCA); 2—OR (optical rectification); 3—CO2 laser frequency mixing; 4—DFG (difference frequency generation); 5—optically pumped laser; 6—QCL (quantum-cascade laser); 7—p-Ge-laser.
Overview of Existing solutions—Existing solutions have limited applications due to their corresponding significant issues:                PCA and OR: low power;        CO2 laser frequency mixing: small spectra range/tunability and low efficiency;        Traditional DFG: relatively low power, low efficiency and complex;        Optically pumped laser: no tunability, gas laser—high maintenance and bulky;        QCL: narrow available spectra range, requires cryogenic cooling; and        p-Ge-laser: narrow available spectra range, operational nightmare—requires a cooling Dewar, and high magnetic and pulsed electric fields.        
U.S. Pat. No. 7,054,339 issued to Yongdan Hu et al. on May 30, 2006, titled “Fiber-laser-based Terahertz sources through difference frequency generation (DFG) by nonlinear optical (NLO) crystals,” is incorporated herein by reference. In the U.S. Pat. No. 7,054,339, Yongdan Hu (one of the inventors of the present invention) and his co-inventors described a fiber-laser-based implementation of a terahertz source through difference frequency generation (DFG) by nonlinear optical (NLO) crystals is compact, tunable and scalable. A pair of fiber lasers (Q-switched, CW (continuous wave) or mode-locked) generate single-frequency outputs at frequencies ω1 and ω2. A fiber beam combiner combines the laser outputs and routes the combined output to a THz generator head where a nonlinear interaction process in the NLO crystal generates THz radiation.
U.S. Pat. No. 7,539,221 issued May 26, 2009 to Jiang, et al., titled “Fiber-laser-based gigahertz sources through difference frequency generation (DFG) by nonlinear optical (NLO) materials,” is incorporated herein by reference. The U.S. Pat. No. 7,539,221 described a fiber-laser-based implementation of a gigahertz source through difference frequency generation (DFG) by nonlinear optical (NLO) materials is compact, tunable and scalable. A pair of pulsed fiber lasers, preferably single-frequency lasers, generate output pulses at frequencies ω1 and ω2 that overlap temporally. A beam combiner combines the laser outputs and routes the combined output to a GHz generator head where a nonlinear interaction process in the NLO material generates GHz radiation.
Optical parametric oscillators (OPOs) provide an efficient way of converting short-wavelength electromagnetic radiation from coherent-light sources to long wavelengths, while also adding the capability to broadly tune the output wavelength. In general, an OPO system principally includes a short-wavelength laser source and an optical resonator (resonant optical cavity) containing a nonlinear crystal. In some embodiments, additional components include mode-matching optics and an optical isolator.
In general, the OPO operates with three overlapping light beams—an input pump beam having the shortest wavelength, and thus highest frequency (typically, this is coherent light from a laser), and two longer-wavelength, lower-frequency beams generated in the OPO called the signal beam (this is usually called the “OPO-signal” beam herein to distinguish from other signals) and the idler beam (this is usually called the “OPO-idler” beam herein). By convention, the shorter-wavelength beam is called the OPO-signal beam, and the longer-wavelength beam is called the OPO-idler beam. The energy of photons in the pump beam (proportional to 1/wavelength) will equal the sum of the energy of photons in the OPO-signal beam plus the energy of photons in the OPO-idler beam. The pump beam (i.e., excitation light from the short-wavelength laser source) is focused, using the mode-matching optics, through the optical isolator and into the resonant optical cavity, passing through the nonlinear crystal(s). Parametric fluorescence generated within the nonlinear material(s) is circulated within the resonant cavity and experiences optical gain. When the OPO is excited by a pump-power-per-unit-area above a certain threshold, oscillation occurs, and efficient conversion of pump photons to OPO-signal and OPO-idler photons occurs. Different configurations of OPOs are possible. Variables include the wavelengths that are resonant within the optical cavity (pump and/or OPO-signal and/or OPO-idler) and the type of resonator (ring versus linear). In a conventional OPO, depending on the application, either the OPO-signal beam or the OPO-idler beam, or both, will be the output light utilized by other components.
U.S. Pat. No. 7,620,077 issued Nov. 17, 2009 to Angus J. Henderson (one of the inventors of the present invention), titled “Apparatus and method for pumping and operating optical parametric oscillators using DFB fiber lasers,” is incorporated herein by reference. In the U.S. Pat. No. 7,620,077, Henderson described an optical parametric oscillator (OPO) that efficiently converts a near-infrared laser beam to tunable mid-infrared wavelength output. In some embodiments, the OPO includes an optical resonator containing a nonlinear crystal, such as periodically-poled lithium niobate. The OPO is pumped by a continuous-wave fiber-laser source having a low-power oscillator and a high-power amplifier, or using just a power oscillator). The fiber oscillator produces a single-frequency output defined by a distributed-feedback (DFB) structure of the fiber. The DFB-fiber-laser output is amplified to a pump level consistent with exceeding an oscillation threshold in the OPO in which only one of two generated waves (“OPO-signal” and “OPO-idler”) is resonant within the optical cavity. This pump source provides the capability to tune the DFB fiber laser by straining the fiber (using an attached piezoelectric element or by other means) that allows the OPO to be continuously tuned over substantial ranges, enabling rapid, wide continuous tuning of the OPO output frequency or frequencies.
U.S. Pat. No. 6,654,392 issued Nov. 25, 2003 to Arbore et al. entitled “Quasi-monolithic tunable optical resonator,” which is hereby incorporated herein by reference, describes an optical resonator having a piezoelectric element attached to a quasi-monolithic structure that defines an optical path. Mirrors attached to the structure deflect light along the optical path. The piezoelectric element controllably strains the quasi-monolithic structure to change a length of the optical path by about 1 micron. A first feedback loop coupled to the piezoelectric element provides fine control over the cavity length. The resonator may include a thermally actuated spacer attached to the cavity and a mirror attached to the spacer. The thermally actuated spacer adjusts the cavity length by up to about 20 microns.
A monolithic resonator typically includes a single block of transparent material having reflecting facets that serve as the mirrors. Usually, the material is strained by changing its temperature. U.S. Pat. No. 4,829,532 issued May 9, 1989 to Kane, which is hereby incorporated herein by reference, describes an alternative where the optical path length of a monolithic oscillator can be adjusted by a piezoelectric element mounted to uniformly strain the entire block in a plane parallel to the plane of the optical path.
U.S. Pat. No. 8,035,083 issued Oct. 11, 2011 to Kozlov et al., titled “Terahertz tunable sources, spectrometers, and imaging systems,” is incorporated herein by reference. Kozlov et al. describe a source of terahertz radiation at a fundamental terahertz frequency that is tunable over a fundamental terahertz-frequency range, and is coupled into a first waveguide. The first waveguide supports only a single transverse spatial mode within the fundamental terahertz frequency range. A solid-state frequency multiplier receives from the first waveguide the terahertz radiation and produces terahertz radiation at a harmonic terahertz frequency. A second waveguide receives the harmonic terahertz radiation. The tunable terahertz source can include a backward-wave oscillator with output tunable over about 0.10-0.18 THz, 0.18-0.26 THz, or 0.2-0.37 THz. The frequency multiplier can include at least one varistor or Schottky diode, and can include a doubler, tripler, pair of doublers, doubler and tripler, or pair of triplers. The terahertz source can be incorporated into a terahertz spectrometer or a terahertz imaging system.
U.S. Pat. No. 7,421,171 issued Sep. 2, 2008 to Ibanescu et al., titled “Efficient terahertz sources by optical rectification in photonic crystals and meta-materials exploiting tailored transverse dispersion relations,” is incorporated herein by reference. Ibanescu et al. describe generating terahertz (THz) radiation. Their system includes a photonic-crystal structure including at least one nonlinear material that enables optical rectification. The photonic-crystal structure is configured to have the suitable transverse dispersion relations and enhanced density photonic states so as to allow THz radiation to be emitted efficiently when an optical or near-infrared pulse travels through the nonlinear part of the photonic crystal.
U.S. Pat. No. 7,473,898 issued Jan. 6, 2009 to Holly et al., titled “Cryogenic terahertz spectroscopy,” is incorporated herein by reference. Holly et al. describe a terahertz spectroscopy system that includes a source of terahertz radiation, a detector of terahertz radiation, a source of sample gas, and a sample cell that can be cooled to cryogenic temperatures. The sample cell may be configured to receive the sample gas, received terahertz radiation from the source of terahertz radiation, provide the terahertz radiation to the detector after the terahertz radiation has passed through the sample gas, and facilitate cryogenic cooling thereof. The sample cell may be cryogenically cooled to freeze the sample gas and subsequently warmed either continuously or in steps in temperature so that individual components or groups of components of the sample gas may evaporate and thus have absorption spectra formed therefor.
U.S. Patent Application Publication US 2005/0018298 of Trotz et al., published Jan. 27, 2005 and titled “Method and apparatus for generating terahertz radiation,” is incorporated herein by reference. Trotz et al. describe generating terahertz radiation. Their terahertz source is described as a versatile terahertz device that can be configured to transmit a plurality of wavelengths, thereby facilitating the detection of multiple contaminants using a single source device. In one embodiment, the Smith-Purcell radiation effect is exploited by passing an electron beam over a modulated conducting surface, wherein the spacing of the periods of the modulated surface is varied. The variations in the modulated surface enable the source to produce light of varying wavelengths.
U.S. Pat. No. 7,781,737 issued Aug. 24, 2010 to Zhdaneev, titled “Apparatus and methods for oil-water-gas analysis using terahertz radiation,” is incorporated herein by reference. Zhdaneev describes analyzing gas-oil-water compounds in oilfield and other applications using terahertz radiation. A sample analyzer includes a sample chamber having a fluid communication port configured to receive the sample. The analyzer also includes a filter to filter samples and selectively remove oil, water or gas from reservoir mixture received by the sample chamber. A terahertz (THz) radiation detector is provided in electromagnetic communication with the sample. The terahertz detector provides a detected output signal indicative of the terahertz electromagnetic radiation detected from the sample. In some embodiments, the device also includes a terahertz source illuminating the sample, the terahertz detector detecting a portion of the terahertz source illumination as modified by the sample. The detected portion of the spectrum of terahertz radiation can be processed to analyze the composition of the sample.
U.S. Pat. No. 7,995,628 issued Aug. 9, 2011 to Wu, titled “Recycling pump-beam method and system for a high-power terahertz parametric source,” is incorporated herein by reference. Wu describes the fabrication of a portable high-power terahertz beam source that can produce what Wu calls a tunable, high-power terahertz beam over the frequency from 0.1 THz to 2.5 THz. Wu's terahertz source employs a recycling pump beam method and a beam quality-control device. The beam quality-control device may or may not be required for a high-power terahertz beam generation. In exemplary embodiments, a lithium niobate (LiNbO3) crystal or a lithium niobate crystal doped with 5% magnesium oxide (LiNbO3:MgO) can be used. Other nonlinear optical crystals, including GaSe can be used in place of the LiNbO3 crystal. Through proper alignment of a pump beam, along with recycling a pump beam, high conversion efficiency is achieved, and a high output power beam is produced at terahertz frequencies.
U.S. Pat. No. 7,391,561 titled “Fiber- or rod-based optical source featuring a large-core, rare-earth-doped photonic-crystal device for generation of high-power pulsed radiation and method,” which issued to Di Teodoro, et al. on Jun. 24, 2008, is incorporated herein by reference. Di Teodoro, et al. describe a photonic-crystal fiber having a very large core while maintaining a single transverse mode. The typical problems of multiple-modes and mode hopping, which result from the use of large-diameter waveguides, are addressed by the invention. By using multiple small waveguides in parallel, large amounts of energy can be passed through a laser, but with better control such that the aforementioned problems can be reduced. An additional advantage is that the polarization of the light can be better maintained as compared to using a single fiber core.
There is still a heretofore unmet need in the art for an improved method and apparatus for high-power fiber-laser-based gigahertz-to-terahertz, millimeter-wave, signal sources for advanced sensors, photonics, and optical computing.